package main.ranking;

/**
 * Hash algorithm by Bob Jenkins, 1996.
 * 
 * You may use this code any way you wish, private, educational, or commercial.
 * It's free. See: http://burtleburtle.net/bob/hash/doobs.html
 * 
 */
public class JenkinsHash {
 
	// max value to limit it to 4 bytes
	private static final long MAX_VALUE = 0xFFFFFFFFL;
 
	// internal variables used in the various calculations
	long a;
	long b;
	long c;
 
	/**
	 * Convert a byte into a long value without making it negative.
	 */
	private long byteToLong(byte b) {
		long val = b & 0x7F;
		if ((b & 0x80) != 0) {
			val += 128;
		}
		return val;
	}
 
	/**
	 * Do addition and turn into 4 bytes.
	 */
	private long add(long val, long add) {
		return (val + add) & MAX_VALUE;
	}
 
	/**
	 * Do subtraction and turn into 4 bytes.
	 */
	private long subtract(long val, long subtract) {
		return (val - subtract) & MAX_VALUE;
	}
 
	/**
	 * Left shift val by shift bits and turn in 4 bytes.
	 */
	private long xor(long val, long xor) {
		return (val ^ xor) & MAX_VALUE;
	}
 
	/**
	 * Left shift val by shift bits. Cut down to 4 bytes.
	 */
	private long leftShift(long val, int shift) {
		return (val << shift) & MAX_VALUE;
	}
 
	/**
	 * Convert 4 bytes from the buffer at offset into a long value.
	 */
	private long fourByteToLong(byte[] bytes, int offset) {
	    return (byteToLong(bytes[offset + 0])
		+ (byteToLong(bytes[offset + 1]) << 8)
		+ (byteToLong(bytes[offset + 2]) << 16) + (byteToLong(bytes[offset + 3]) << 24));
	}
 
	/**
	 * Mix up the values in the hash function.
	 */
	private void hashMix() {
		a = subtract(a, b);
		a = subtract(a, c);
		a = xor(a, c >> 13);
		b = subtract(b, c);
		b = subtract(b, a);
		b = xor(b, leftShift(a, 8));
		c = subtract(c, a);
		c = subtract(c, b);
		c = xor(c, (b >> 13));
		a = subtract(a, b);
		a = subtract(a, c);
		a = xor(a, (c >> 12));
		b = subtract(b, c);
		b = subtract(b, a);
		b = xor(b, leftShift(a, 16));
		c = subtract(c, a);
		c = subtract(c, b);
		c = xor(c, (b >> 5));
		a = subtract(a, b);
		a = subtract(a, c);
		a = xor(a, (c >> 3));
		b = subtract(b, c);
		b = subtract(b, a);
		b = xor(b, leftShift(a, 10));
		c = subtract(c, a);
		c = subtract(c, b);
		c = xor(c, (b >> 15));
	}
 
	/**
	 * Hash a variable-length key into a 32-bit value. Every bit of the key
	 * affects every bit of the return value. Every 1-bit and 2-bit delta
	 * achieves avalanche. The best hash table sizes are powers of 2.
	 * 
	 * @param buffer
	 *            Byte array that we are hashing on.
	 * @param initialValue
	 *            Initial value of the hash if we are continuing from a previous
	 *            run. 0 if none.
	 * @return Hash value for the buffer.
	 */
	public long hash(byte[] buffer, long initialValue) {
		int len, pos;
 
		// set up the internal state
		// the golden ratio; an arbitrary value
		a = 0x09e3779b9L;
		// the golden ratio; an arbitrary value
		b = 0x09e3779b9L;
		// the previous hash value
 
		//c = initialValue;
		c = 0x0E6359A60L;
 
		// handle most of the key
		pos = 0;
		for (len = buffer.length; len >= 12; len -= 12) {
			a = add(a, fourByteToLong(buffer, pos));
			b = add(b, fourByteToLong(buffer, pos + 4));
			c = add(c, fourByteToLong(buffer, pos + 8));
			hashMix();
			pos += 12;
		}
 
		c += buffer.length;
 
		// all the case statements fall through to the next on purpose
		switch (len) {
		case 11:
			c = add(c, leftShift(byteToLong(buffer[pos + 10]), 24));
		case 10:
			c = add(c, leftShift(byteToLong(buffer[pos + 9]), 16));
		case 9:
			c = add(c, leftShift(byteToLong(buffer[pos + 8]), 8));
			// the first byte of c is reserved for the length
		case 8:
			b = add(b, leftShift(byteToLong(buffer[pos + 7]), 24));
		case 7:
			b = add(b, leftShift(byteToLong(buffer[pos + 6]), 16));
		case 6:
			b = add(b, leftShift(byteToLong(buffer[pos + 5]), 8));
		case 5:
			b = add(b, byteToLong(buffer[pos + 4]));
		case 4:
			a = add(a, leftShift(byteToLong(buffer[pos + 3]), 24));
		case 3:
			a = add(a, leftShift(byteToLong(buffer[pos + 2]), 16));
		case 2:
			a = add(a, leftShift(byteToLong(buffer[pos + 1]), 8));
		case 1:
			a = add(a, byteToLong(buffer[pos + 0]));
			// case 0: nothing left to add
		}
		hashMix();
 
		return c;
	}
 
	/**
	 * See hash(byte[] buffer, long initialValue)
	 * 
	 * @param buffer
	 *            Byte array that we are hashing on.
	 * @return Hash value for the buffer.
	 */
	public long hash(byte[] buffer) {
		return hash(buffer, 0);
	}
}